1. Field of the Invention
The present techniques relate generally to the formation of plastic bottles and other plastic articles. Specifically, the present techniques relate to the production of plastic bottles and articles having good environmental stress crack resistance and a high gloss outer surface.
2. Description of the Related Art
This section is intended to introduce the reader to various aspects of art which may be related to various aspects of the present invention that are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
As chemical and petrochemical technologies have advanced, the products of these technologies have become increasing prevalent in society. In particular, as techniques for bonding simple molecular building blocks into long chains, or polymers, have advanced, polymer products have been increasingly incorporated into various everyday items. For example, polymers such as polystyrene, polyethylene, polypropylene, and various copolymers, are used for retail and pharmaceutical packaging, food and beverage packing, household containers, household items, automobile components, pipes, conduits, toys, and various industrial products.
Specific types of polymers, such as high density polyethylene (HDPE), have particular applications in the manufacture of blow molded and injection molded goods, such as food and beverage containers, film, and plastic pipes. HDPE provides a number of advantages for these applications, for example, resistance to an effect known as environmental stress cracking. Environmental stress cracking is the degradation of a plastic product in contact with a particular material or solvent. When the product in contact with the problem material is placed under stress, it may result in cracking of the polymer leading to failure. This failure may result in the release of the contents.
Further, HDPE is generally a strong and stiff material with low oxygen and moisture vapor transmission, and thus provides high barrier properties. The combination of environmental stress crack resistance, strength, stiffness, raw material cost and barrier properties makes HDPE a suitable material for packaging containers (e.g., bottles) for soap, milk, or other materials that need high protection from the environment or that may cause stress cracking of other types of polymers. However, HDPE typically has the disadvantage that formed products have a low surface gloss. For many applications, the low surface gloss decreases the brilliance and attractiveness of the marketing materials on the exterior of the HDPE bottle.
Containers may be constructed of other polymers, such as those made using large amounts of styrene, to provide a higher surface gloss. Unfortunately, however, styrenic copolymers may have significant disadvantages when used as containers for particular materials. For example, many styrenic copolymers are sensitive to environmental stress cracking and cannot be used for such applications as soap, which may cause the container to fail. Further, such copolymers may not have the moisture barrier properties to protect some materials from evaporative losses over time. For example, high water content materials, such as aqueous solutions, may experience noticeable product loss over a shorter period of time when kept in a bottle made from a styrenic copolymer versus a bottle made from HDPE.
A number of techniques have been tested to improve the surface gloss of products made from HDPE, such as bottles. These techniques have included coextrusion with other polymers that provide a higher surface gloss. Examples of higher gloss polymers tested include specialty medium density polyethylene (MDPE) grades. However, in addition to having a lower gloss than a polymer made with a large amount of styrene, products made having an outer surface of MDPE may have a waxy surface and problems with label adhesion, for example.
Styrenic copolymers have also been tested in coextruded applications to improve the gloss of HDPE products. However, these polymers have increased the surface friction of the products, leading to problems with the products feeding through the machinery and equipment required for filling, labeling, or other processing operations. Further, in many cases, these polymers may be sensitive to scratching, leading to poor surface appearance when the blow molded article reaches the marketing outlet for sale to the end consumer.